This invention relates to the recovery of constituents from subterranean formations, and more particularly to an in situ method of recovery that is particularly effective for the production of shale oil from oil shale in an in situ oil shale retort. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen" which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid product is called "shale oil."
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which in U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean formation containing oil shale by undercutting a portion of the subterranean formation leaving an overlaying deposit supported by a multiplicity of pillars. The pillars are removed, e.g., with explosive, and overlying deposit is expanded to provide a permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying combustion zone feed into the retort on the trailing side of the combustion zone to advance the combustion zone through the fragmented mass. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue of solid carbonaceous material. The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniformly fragmented, reasonably uniformly permeable mass of formation particles having a reasonably uniformly distributed void volume or void fraction so gases can flow uniformly through the retort and result in maximum converstion of kerogen to shale oil. A uniformly distributed void fraction in the direction perpendicular to the direction of advancement of the combustion zone is important to avoid channeling of gas flow in the retort. The creation of a mass of particles of uniform void volume distribution prevents the formation of over-sized voids or channels which hinder total recovery of shale oil and also provides a uniform pressure drop through the entire mass of particles. In preparation for the described retorting process, it is important that the formation and the support pillars be fragmented and displaced, rather than simply fractured, in order to create high permeability; otherwise, too much pressure differential is required to pass gas through the retort.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed. The overlying formation is then expended by detonating explosives placed in the formation to fill the void created by the undercut.
The general art of blasting rock formations is discussed in The Blaster' Handbook, 15th Edition, published by E. I. duPont de Nemours & Company, Wilmington, Delaware.
U.S. Pat. No. 3,434,757 issued to Prats describes detonation of explosive in arches between parallel tunnels in oil shale to create a large unsupported roof area that collapses into the tunnels. Additional formation is fragmented by sequential detonation of a series of explosives to form permeable zones in the oil shale, and hot fluid is passed through the permeable zones for producing shale oil.
Other methods for preparing formation for in situ recovery are described in U.S. Pat. Nos. 4,043,597 and 4,043,598, both assigned to the assignee of this invention, and both incorporated herein by this reference. According to these patents, at least two voids vertically spaced apart from each other are excavated in the subterranean formation. This leaves a zone of unfragmented formation between adjacent voids. Explosive is placed in blasting holes and detonated to expand formation in the intervening zone towards both voids.
U.S. Pat. No. 3,980,339 to Heald et al describes mining out an area at the base of an oil shale deposit leaving overlying deposit supported by a plurality of pillars. The pillars are removed with explosive and overlying formation is expanded into the underlying area with explosives. The disclosure of this patent, particularly that relating to the removal of support pillars with explosive, is incorporated herein by this reference.
Copending U.S. Patent Application Ser. No. 833,240 filed Sept. 14, 1977, by Gordon B. French, titled EXPLOSIVE PLACEMENT FOR EXPLOSIVE EXPANSION TOWARD SPACED APART VOIDS, which is assigned to the Assignee of the present application, describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids containing support pillars. The pillars are explosively expanded to spread the particles thereof uniformly across the void, and unfragmented formation adjacent the void is explosively expanded toward the void before overlying, unsupported formation can cave into the void. Said U.S. Patent Application Ser. No. 833,240 is incorporated herein by this reference.